The function of platelets is to repair small blood vessels and prevent dangerous amounts of blood from leaking out. When a person is cut, platelets rush to the area and cling to the blood vessels that have been damaged, sealing them. Platelets were once called thrombocytes.
When the platelets gather at a cut blood vessel, they give off several chemicals called platelet factors. These chemicals combine with clotting factors in the blood plasma to make thromboplastin. Plasma is the yellowish, liquid portion of the blood.
Thromboplastin then interacts with prothrombin and other chemicals to make thrombin. Thrombin combines with fibrinogen to make long threads of fibrin, which creates a mesh in the wound to hold the platelets and other blood cells. This basically forms a plug that stops the bleeding.
Platelets are tiny, flexible, sticky blobs that can resemble plates when they are not in use. When they react to a cut, they change shape and can grow long fibers. They are made in the bone marrow out of cytoplasm. They're much smaller than either red or white blood cells, and a healthy human being has between 150,000 to 300,000 platelets per microliter of blood. Having too high or too low an amount of platelets can lead to health problems.